Ethylene-propylene copolymers are important commercial products and are widely used as viscosity modifiers (VM) in lubricating oils. A motor oil should not be too viscous at low temperatures so as to avoid serious frictional losses, facilitate cold starting, and provide free oil circulation at engine start-up. On the other hand, too thin an oil at high temperature will cause excessive engine wear and oil consumption. It is most desirable to employ a lubricating oil which experiences little or no viscosity change in response to changes in oil temperature.
Over the last thirty years, efforts have been made to improve the thickening efficiency (TE) and shear stability (SSI) performance of lubricating oil viscosity modifiers. The thickening efficiency is a measure of the thickening power of the polymer, and is defined as: EQU TE=(2/C) ln ((kv of polymer+oil)/(kv of oil))/ln (2)
wherein kv is the kinematic viscosity at 100.degree. C., C is the concentration in grams/100 grams of solution, and the log is consistently either natural or base 10.
The TE of a viscosity modifier depends somewhat on the particular base oil and other formulating components in the base oil, as well as the polymer concentration. For the purposes of this application, the oil is designated to be a solvent 100N, such as FTN135 (a product of Exxon Chemicals) containing no components other than the viscosity modifier of the present invention, with a polymer concentration sufficient to double the viscosity of the base oil at 100.degree. C.
The shear stability is a measure of how well the polymer resists degradation due to the mechanical stresses applied by an engine. The SAE-ASTM-DIN test used to measure this tendency for degradation is the Kurt Orbahn Shear Stability Test (ASTM D3945-86, hereinafter "KO SSI").
Previous efforts to simultaneously improve TE and SSI of ethylene, .alpha.-olefin copolymers have involved increasing the ethylene content of the polymer and narrowing the molecular weight distribution (MWD). The upper limit for ethylene content is determined by the requirement that the dilute polymer has to be soluble, or remain in suspension with no macroscopic gellation,in oil down to a temperature of at least -40.degree. C. Thus, pure polyethylene is beyond the limit. For simple statistical copolymers, the average ethylene content cannot be made higher than about 76 wt. % (determined in accordance with ASTM D3900-95) without causing the copolymer to become insoluble in oil. At these levels, however, pour point problems are encountered. However, it has been demonstrated that even when the ethylene content exceeds only about 61 wt. %, many of the methylene sequences in the viscosity modifier are of a length which allows them to co-crystallize with paraffin waxes in the oil, thereby leading to undesirably high viscosities and gellation of the oil. Typically, conventional ethylene, .alpha.-olefin viscosity modifiers that can be employed to formulate lubricating oils having pour points of -30.degree. C. or below are restricted to an average ethylene content of no greater than about 56 wt. %, unless the molecules possess an intramolecular compositional distribution (CD) tailored to improve such properties, as is disclosed in U.S. Pat. No. 4,900,461.
The present inventors have developed a unique viscosity modifier, formed of a block copolymer comprising a first block of substantially pure polyethylene and a second block of a copolymer of ethylene and another .alpha.-olefin; such that the block copolymer has a high average ethylene content so as to provide an excellent balance of thickening efficiency and shear stability. More importantly, however, is the fact that this unique block copolymer viscosity modifier of the present invention possesses unusual solubility characteristics, agglomerating above wax crystallization temperatures, thereby preventing its co-crystallizing with paraffin waxes in the oil and the undesirable formation of high viscosities or gellation of the oil. Furthermore, the present inventors have discovered that when blocks of substantially pure ethylene are used as the A block of a block copolymer, the ethylene sequences in the B block, even in the 50 wt. % range of B block ethylene content, will not crystallize with paraffins and cause high oil viscosity at temperatures below about 0.degree. C. This allows for an increased ethylene content in the B blocks, an average ethylene content in the block copolymer of over 70 wt. % and performance characteristics that are substantially independent of the type of wax contained in the lubricant basestock.
In summary, these viscosity modifiers exhibit excellent TE-SSI performance, and can be added to the oils to form a viscosity modified lubricant with excellent low temperature properties.
The present invention also provides a polymer with resistance to cold flow during storage and resistance to agglomerization during recovery from the polymerization solvent and many additional advantages which shall become apparent as described below.